Recording medium

ABSTRACT

A rewritable or repeatedly employable recording medium having a base and a recording layer film provided on said base, characterized in that said recording layer film is formed by a thermotropic cholesteric liquid crystal polyester.

This application is a continuation of application Ser. No. 909,484,filed Sept. 19, 1986, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a novel recording medium suitable for athermal recording of image, characters, etc.

Recently, with a rapid progress of opto-electronic system, there hasbeen popularly used a method in which information optically-read orstored in a memory are converted to such an output as heat or laser beamand recorded on a recording medium. For example, a signal is convertedto a laser beam, then the laser beam is applied to a recording mediumhaving an amorphous metal or a photosensitive agent and is recordedthereon by a physical change or discoloration of the recording medium.

In such conventional method, however, it is necessary to go through anexpensive and complicated process such as vapor metalizing and the useof an expensive photosensitive agent is essential, thus resulting in agreatly increased production cost.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide a novel recordingmedium free of the abovementioned drawbacks and particularly provide anovel recording medium which permits the use of a relatively inexpensivematerial and can be produced in a simple manner. The recording is madeby a contrast between the portion of a recording layer which has beenrendered transparent by heat and an opaque portion, further, the storageof recorded information is semipermanent and it is possible to effectrewriting.

The present invention relates to a recording medium having a recordinglayer formed by a thermotropic cholesteric liquid crystal polyester.More particularly, it is concerned with a rewritable or repeatedlyemployable recording medium having a base and a recording layer filmformed thereon, in which the recording layer film is formed by athermotropic cholesteric liquid crystal polyester, and at roomtemperature or thereabouts the said polyester is in a polydomain stateor crystallized state and is opaque, and by melting in a liquid crystalforming temperature region said polydomain state or crystallized stateis transformed into a monodomain state, and by cooling rapidly andmaintaining the monodomain state at room temperature, the recordinglayer film can be made transparent.

DETAILED DESCRIPTION OF THE INVENTION

As the base in the recording medium of the present invention there maybe used, for example a polycarbonate, polyether imide or polyimide filmor sheet, or a glass plate, or the said film or sheet plus a reflectivelayer formed thereon by vapor metalizing or plating, or a film or sheetof a metal having a mirror surface.

The recording layer film on the base is a film of a thermotropiccholesteric liquid crystal polyester, which is formed on the base in athickness of 1 to 100 μm by a conventional method. The recording mediummay be further covered with a transparent film as a protective film.

The cholesteric liquid crystal polyester used in the present inventionis prepared by copolymerizing a nematic or smectic liquid crystalpolyester with an optically active component. Such nematic or smecticliquid crystal polyester is an aromatic polyester usually having, asmesogens, structural units selected from the following structural unitsderived from aromatic diols and those derived from aromatic dicarboxylicacids or cyclohexanedicarboxylic acids.

As structural units derived from aromatic diols: ##STR1## wherein X ishydrogen, halogen, or an alkyl group having not more than four carbonatoms, ##STR2##

As structural units derived from aromatic dicarboxylic acids orcyclohexanedicarboxylic acids: ##STR3## wherein X is hydrogen, halogen,or an alkyl group having not more than four carbon atoms, ##STR4##

Or it is a polyester usually having, as mesogens, structural unitsselected from the following structural units derived from aromatichydroxycarboxylic acids: ##STR5## wherein X is hydrogen, halogen, or analkyl group having not more than four carbon atoms, ##STR6##

The cholesteric liquid crystal polyester is prepared by copolymerizingthe aromatic polyester having such structural units as mesogens with anoptically active components. As the optically active component there isused an optically active compound which is copolymerizable with suchstructural units. Selecting a bifunctional compound as the opticallyactive component is desirable in that it can be introduced in anydesired amount into the polymer chain. Even a monofunctional compoundwhich can be introduced only in polymer ends is also employable in thepresent invention if it exhibits a cholesteric liquid crystal mesophase.

As examples of bifunctional optically active monomers there arementioned dicarboxylic acids such as (R)- or (S)-3-methyladipic acid andbenzoylglutamic acid; (R)- or (S)-diols such as 1,2-propanediol and2-methyl-1,4-butanediol; and (R)- or (S)-hydroxy acids such as3-hydroxybutanoic acid and 2-methyl-3-hydroxypropionic acid. These maybe used each alone or as a mixture in the copolymerization. Opticalpurity need not always be 100%, but in the case of a mixture a helicalpitch in cholesteric liquid crystal becomes large as compared with thatfrom a pure (R) or (S) form and the cholesteric mesophase formingability is deteriorated. Generally, the difference between the contentof (R) form and that of (S) form should be not less than 15%, preferablynot less than 30%.

The optically active component is present at a proportion of 1 to 10mol%.

The cholesteric liquid crystal polyester used in the present inventionmay incorporate a further unit exemplified below for the adjustment ofits melting point:

Monomer units such as m-substituted benzene derivatives and2,7-substituted naphthalene derivatives, e.g.: ##STR7##

Monomer units containing a freely rotatable group between aromatic ringssuch as those represented by the following general formulae: ##STR8##wherein X is O, CH₂, C(CH₃)₂, or SO₂.

Units derived from aliphatic diols and aliphatic dicarboxylic acidsrepresented by the following general formulae: ##STR9## wherein m and nare each an integer of 2 to 12.

The cholesteric liquid crystal polyester used in the present inventioncan be prepared by a conventional method. For example, a diacid chloridecondenses with a diol while allowing dehydrochlorination to take place;or a dicarboxylic acid dimethyl ester or diphenyl ester condenses with adiol at a high temperature under a high vacuum; or a dicarboxylic acidcondenses with a diacetylated diol at a high temperature under a highvacuum; or a dicarboxylic acid condenses directly with a diol in thepresence of a condensing agent such as a phosphorus compound. In thecase of using a hydroxycarboxylic acid compound with the monomersmentioned above, a copolycondensation can be performed according to theabove polymerization method.

The cholesteric liquid crystal polyester used in the present inventionis formed into a film of 1 to 100 μm, preferably 2 to 50 μm, inthickness and heat-melted in a liquid crystal forming temperature regionranging from 150° to 350° C., preferably 200° to 300° C., whereby it canbe changed from a polydomain state to a monodomain state or a state akinto monodomain. By a rapid cooling from the liquid crystal formingtemperature region down to a temperature below the crystallizationtemperature [or below Tg temperature (glass transition temperature) ifcrystallization does not take place] a monodomain state can be frozen.

The term "polydomain" represents a state as followed; constituentdomains of a cholesteric liquid crystal are so small in size and thedirection of helical axes are so randomly oriented in a film, that lightis scattered at crystal interfaces to afford an opaque film. On theother hand, the term "monodomain or ordered planar texture represents astate in which the helical axis of cholesteric liquid crystal isperpendicular to the film plane and the cholesteric laminate structurespreads throughout the whole surface of film like an ideal singlecrystal as a result of uniting polydomains in a heat-melted condition.In this case, if the helical axis pitch length is close to thewavelength of visible light, a light corresponding to the wavelengthλ_(o) is reflected selectively in accordance with the followingequation:

λ_(o) =nP

wherein n and P represent an average refractive index and a helical axispitch length, respectively, and the film looks colored. If λ_(o)deviates from the visible light wavelength, the film looks colorless andtransparent.

Conversely, the change from transparent to opaque state is attained bycooling molten cholesteric liquid crystal slowly to let crystallizationtake place. It is also possible to obtain a polydomain opaque state byapplying an electric or magnetic field to liquid crystal in a moltenstate to disorder the orientation of liquid crystal followed by coolingrapidly in the disordered state. Thus, by using the cholesteric liquidcrystal polyester of the present invention it is made possible to effectthe change from transparent to opaque state, and vice versa, reversibly;that is, the said polyester is suitable for use as a writable anderasable recording medium.

The melting of the cholesteric liquid crystal polyester is attained byapplying a heat source such as laser beam, xenon lamp light or a highfrequency to a required portion.

Recorded information are taken out by the difference in transmission oflight or by the difference in reflectivity of light or any othersuitable means.

On the basis of such principle the recording medium of the presentinvention can be utilized as an optical disc memory or a photo mask fora printing plate or electronic circuits.

EXAMPLES

The following examples are given to illustrate the invention in moredetail.

EXAMPLE 1

15.55 g of polyethylene terephthalate having an inherent viscosity (ηinh) of 0.37 dl/g as measured using a mixed solvent ofphenol/tetrachloroethane (60/40 by weight) at 30° C. at a concentrationof 0.5 wt.% (the following η inh values were also measured according tothis method), 34.02 g of p-acetoxybenzoic acid--, 4.33 g of(R)-3-methyladipic acid and 5.24 g of diacetylated hydroquinone werecharged into a polymerization flask equipped with a stirrer, then afterpurging with nitrogen, the polymerization flask was dipped in an oilbath held at 220° C. and reaction was allowed to take place for 1 hourin a nitrogen atmosphere under stirring, then the temperature was raisedto 230° C. and reaction was allowed to proceed for another one hour. At230° C. nitrogen was started to flow and after one hour, the reactionwas allowed to proceed for further one hour under a reduced pressure.Then the reaction temperature was raised from 230° C. to 270° C. over aperiod of about 30 minutes, and at 270° C. the degree of vacuum was heldat 0.2 mmHg for 1 hour to complete the polymerization. As a result,there was obtained a polymer having η inh=0.45 dl/g.

The polymer was dissolved in p-chlorophenol to prepare a 10 wt.%solution. This solution was applied onto an 80 um polyimide film bymeans of a roll coater and dried for 12 hours at room temperature andfor 3 hours at 100° C. under vacuum to obtain a recording medium havinga thickness of 6.5 μm. The transmission of the recording medium wasmeasured using a monochromatic light having a wavelength of 800 nmthrough a spectrophotometer. When the recording medium was heated to250° C. and then cooled rapidly, its transmission was found to be 90.5%.The recording medium was again heated to 250° C. and then cooled slowlyat a rate of 20° C./min; as a result, its transmission was 24.5%. Whenit was again heated to 250° C. followed by cooling rapidly, there wasobserved a recovery of transmission to 90.1%. The degree of transparencyand that of opaqueness did not change at all even after the lapse ofthree months at room temperature.

EXAMPLE 2

Polymerization was performed under the same conditions as in Example 1except that the amounts of polyethylene terephthalate, p-acetoxybenzoicacid, (R)-3-methyladipic acid and diacetylated hydroquinone were changedto 7.68 g, 32.40 g, 3.20 g and 3.84 g, respectively, to afford a polymerhaving η inh of 0.48 dl/g. The polymer was applied onto a polyimide filmin the same way as in Example 1 to obtain a recording medium having athickness of 6.5 μm. When this recording medium was heated to 290° C.and then cooled rapidly, its transmission was found to be 90.1%. Therecording medium was again heated to 290° C. and then cooled slowly at arate of 20° C./min; as a result, its transmission was 21.3%. When it wasagain heated to 290° C. followed by cooling rapidly, there was observeda recovery of transmission to 92.3%. The degree of transparency and thatof opaqueness did not change at all even after the lapse of three monthsat room temperature.

EFFECT OF THE INVENTION

The recording medium of the present invention can create bothtransparent and opaque portions under the supply of heat; besides, thechange from one to another is reversible and a semipermanent storage ofrecorded information can be effected at room temperature or thereabouts.Further, since recorded information can be taken out using light, therecording medium of the present invention is suitable for use as anoptical disc memory or a photo mask.

What is claimed is:
 1. A rewritable or repeatably employable recordingmedium comprising a base and a recording layer film deposited on saidbase; said recording layer film being a film of a thermotropiccholesteric liquid crystal polyester; said recording layer film beingreversibly transparent or opaque, said film being transparent at roomtemperature by heat-melting said film at the liquid crystal formingtemperature followed by rapid cooling from said liquid crystal formingtemperature to room temperature and being opaque at room temperature byheat-melting said film at the liquid crystal forming temperaturefollowed by either (a) slow cooling to room temperature or (b) theapplication of an electrical or a magnetic field followed by rapidcooling to room temperature; said cholesteric liquid crystal polyesterof said recording layer film formed by copolymerizing an opticallyactive compound with a mesogen, said mesogen have a structural unitselected from the group consisting of ##STR10## the combination of##STR11## and the combination of ##STR12## where X and Y are eachindependently hydrogen, halogen or an alkyl group having no more thanfour carbon atoms.
 2. The recording medium of claim 1, wherein saidthermotropic cholesteric liquid crystal polyester has a liquid crystalforming temperature in the range of 150° to 350° C.
 3. The recordingmedium of claim 1 wherein said recording layer film has a thickness inthe range of 1 to 100 μm.
 4. The recording medium of claim 1, wherein(R)- or (S)-3- methyladipic acid is copolymerized with the mesogen. 5.The recording medium of claim 1, wherein (R)- or(S)-2-methyl-1,4-butanediol is copolymerized with the mesogen.
 6. Therecording medium of claim 4, wherein said optically active component ispresent in a proportion of 1 to 10 mol% of all monomer units.